1. Field of the Invention
The present invention relates to a process for the selective deposition of thin diamond films by gas phase synthesis, and specifically to a process for selectively growing by gas phase synthesis thin diamond films on desired areas of a silicon substrate or on desired areas of a surface of a basal thin diamond film formed beforehand on a desired substrate by gas phase synthesis. This process is useful for the development of new devices and materials in the semiconductor field, the Electronic fields, and the like.
2. Discussion of the Background
It has been known that a thin diamond film can be formed on a substrate by gas phase synthesis, namely, by using as a raw material a gas mixture of a hydrocarbon gas, such as methane, and a hydrogen gas, and by subjecting it to cracking or plasma reaction in a vacuum chamber. A variety of processes has been proposed to practice this technique. It has now been recognized by identification characterization of various properties such as crystallinity, the directionality of crystalline growth and thermal conductivity that various thin diamond films so formed by gas phase synthesis have properties identical to natural diamond.
As examples of potential application fields making use of such properties of thin diamond films formed by gas phase synthesis, various attempts have heretofore been made to use them as abrasive coatings for cemented carbide (WC) tools, heat sinks for ICs, coatings for speaker diaphragms and so on. In such applications, it is required to deposit thin diamond films uniformly on substrates or base materials. Accordingly, selective deposition has not been attempted at all.
As a further example of an application field of the thin film technology, there is the semiconductor field. This field is considered to have the greatest future prospect and development as an application field of thin diamond films. Current semiconductors are mainly Si semiconductors which employ a silicon wafer as a substrate. If it becomes feasible to form a thin diamond film on a silicon wafer and further to form the thin diamond film as a fine pattern on the order of micrometers by selective deposition, it will be possible to use thin diamond films as heat sinks for certain specific elements of integrated circuits, as insulators between circuit wirings in integrated circuits, in high performance devices fabricated by hybridization of Si and thin diamond films, as optical waveguides using thin diamond films, and the like.
Under the current level of technology for depositing a thin diamond film on a silicon substrate, it is essential as shown in FIGS. 4(a) to 4(c) to abrade a surface of a silicon wafer 1 with diamond powder or paste of a particle size on the order of several micrometers to provide an abraded wafer 1' having a number of scratches 21 in the surface and then to deposit a thin diamond film 22 thereon by gas phase synthesis. Owing to the existence of the surface scratches 21, the diamond nucleation density is increased substantially so that the thin diamond film 22 can be obtained as a continuous polycrystalline thin diamond film. If the above abrasion were not applied, no diamond would practically be formed on the silicon wafer. On the contrary, the abrasion results in uniform formation of diamond particles on the surface of the silicon wafer, whereby diamond eventually grows into a continuous film. It is therefore impossible to form a circuit or pattern, which has a desired configuration, with a thin diamond film.
As is known very well, diamond has the highest hardness among all the materials and also has a high thermal conductivity and excellent heat resistance and radiation resistance. Using gas-phase-synthesized diamond, the development of environment-resistant electronic devices, semiconductor devices and the like, which are equipped with such attractive properties, are now under way.
For the application of gas-phase-synthesized diamond in the field of such electronic technology, there has been an outstanding demand for the development of a technique for permitting selective formation or deposition of thin diamond films on desired areas of a surface of silicon substrate or wafer. With a view toward meeting this demand, the present assignee has proposed the following process for the gas phase synthesis of diamond (Japanese Patent Application Laid-Open No. 297298/1987).
Namely, prior to gas phase synthesis of diamond on a silicon substrate, desired areas of a surface of the substrate, on which diamond is to be deposited, are selectively covered with a masking material. The remaining areas other than those covered with the masking material are coated with an amorphous material such as amorphous silicon. The masking material is then stripped off to expose the surface of the substrate, followed by the application of gas phase synthesis thereby to deposit a thin diamond film on the thus-exposed surface areas of the substrate.
Since the above technique has made it possible to form a thin diamond film on a surface of a silicon substrate in accordance with a desired pattern, trial fabrication of diamond semiconductors and the like has been further facilitated.
The above technique has however been found to involve such problems that when a silicon substrate is used as a basal substrate, the maximum breakdown voltage of the resulting device is governed by silicon as the basal substrate and the device is prone to damages due to the difference in thermal expansion coefficient between silicon and diamond when used at elevated temperatures.
If a technique can be established to permit the deposition of an additional thin diamond film in accordance with a desired pattern by gas phase synthesis on a surface of a basal thin diamond film formed beforehand on a silicon substrate by gas phase synthesis, the above-described problems can be solved by etching off the silicon substrate alone, if necessary, after forming a semiconducting device on the basis of such a technique.